With increasing emissions regulations, the demand for more fuel efficient vehicles has also increased. In order to increase fuel economy, the prevalence of technologies such as automatic start-stop systems are on the rise. These start-stop systems automatically shut down the engine when the vehicle is stopped and idling, a major source of fuel waste, by withholding fuel delivery to the combustion chambers. Prolonged idle conditions are typical for delivery vehicles, taxis, and commuters in heavy stop-and-go traffic. During temporary engine shut down, as soon as the vehicle controller receives a torque demand from the operator, or an indication of other engine parameters (e.g., low battery charge) necessitating engine power, the engine is restarted and resumes nominal operation. Automatic engine start-stop systems are present in many hybrid electric vehicles, where supplemental electric motors are utilized to run auxiliary systems when the engine is off. Many hybrid electric vehicles employ the use of an integrated starter-motor that replaces the conventional starter motor and alternator for the purpose of capturing regenerative braking energy and withstanding the frequent number of start-stop cycles inherent to these systems.
Hydraulic hybrid vehicles (HHV) may store pressurized fluid in a reservoir (e.g., accumulator) during engine operation. One example of hydraulic hybrid vehicle is shown by Teslak, et al. in U.S. Pat. No. 7,146,266. Therein, a reversible pump uses braking energy to direct hydraulic fluid into a nitrogen-filled high pressure accumulator. When a vehicle controller receives a torque demand from the operator, the pump is reversed and the pressurized fluid is used to accelerate the vehicle. The inventors, however, have recognized potential issues with such systems. In one example, the hydraulic brake systems associated with compression-ignition engines are typically supplied with hydraulic pressure from a primary engine source, such as the power steering system, which does not function when the engine is off. In order to maintain functionality of the brake system during an engine-off condition, these systems often include a supplemental electric pump motor.
Attempts to address integrating automatic start-stop systems into compression-ignition engine systems include what is known as a full hybrid, where a hydraulic pump-motor is coupled to the engine for starting and stopping both the vehicle and the engine. One example approach is shown by Beaty, et al. in U.S. Pat. No. 7,104,920. Therein, a hydraulic hybrid powertrain system includes an engine coupled to a hydraulic pump-motor, the hydraulic pump-motor coupled to a transmission of the vehicle and to a hydraulic accumulator. The transmission may be operated via the hydraulic pump-motor, or by both the engine and hydraulic pump-motor. The inventors, however, have recognized potential issues with such full hybrid systems. In one example, the systems above require the addition of a hydraulic pump-motor of significant size, which adds substantial cost, weight, and complexity to the vehicle system, making it impractical to implement on most passenger vehicles.
In one example, the issues described above may be addressed by a method for a vehicle, including: in response to the vehicle coming to a stop, supplying pressure to a hydraulic braking system of the vehicle from an accumulator coupled to a hydraulic pump coupled to a shaft of a turbocharger (e.g., hydraulic turbocharger) of an engine installed in the vehicle, and automatically shutting down the engine while the vehicle is stopped. In this way, the hydraulic pressure accumulator coupled to a hydraulic hybrid engine may be used as a brake energy source to enable to the use of automatic start-stop technology, thereby improving fuel economy and reducing emissions.
As one example, during engine operation, a hydraulic pump coupled to the rotating shaft of the turbocharger charges the accumulator (e.g., hydraulic accumulator) with hydraulic pressure when the pressure in the accumulator is less than a threshold. When the vehicle comes to a stop for more than a threshold duration, and the pressure in the accumulator is above a threshold, the brake fluid supplied to a brake assembly coupled to a wheel of the vehicle will transition from being sourced from a primary engine source to being sourced from a hydraulic accumulator. The hydraulic accumulator is capable of maintaining hydraulic pressure even when the engine is off, unlike typical hydraulic brake systems associated with compression-ignition engines, where the primary engine source loses power, and consequently hydraulic pressure, when the engine is shut off. As a result of being able to utilize the accumulator to supply pressurized hydraulic brake fluid to the brake system, automatic engine start-stop may be enabled for a compression-ignition engine. In this way, the use of an expensive pump-motor to maintain hydraulic pressure may be avoided, keeping manufacturing costs down and increasing fuel efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.